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While I cannot say for sure, I believe duration the ground is frozen and duration at extreme temperatures probably plays the biggest effect. Especially wind at those conditions. Here in Wisconsin, even a warm winter the chances of over wintering Rosemary is slim to none.

My hardy rosemary (can't remember if it was arp or hill hardy) didn't make it planted outdoors last winter. It is very easy to start plants from cuttings and keep them going indoors. I use soil to root. Their enemy is dry hear so steer clear of that and you will have plenty of rosemary to tend to this winter.

I've never had success keeping the rosemary sold as a christmas tree alive. As soon as it leaves Lowes its a gonner. It is also labeled not for culinary use.

In over 10 years I've had one rosemary overwinter once in the garden - it must have been an amazing combination of snowfall and temperature and will probably never happen again. I always keep 3 of the plants in a large (and heavy) pot in the driveway. It is perfectly happy until about -10 C, below that and I drag it into the garage, putting it back out if there is a stretch of warmer weather. When the weather turns permanently cold, it stays in the unlighted garage and occasionally gets a few handfuls of snow for moisture. Back outside when the weather warms up and it's good to go.

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